1. Field of the Invention
This document relates to a liquid crystal display, and more particularly, to a liquid crystal display of a fringe field switching type which can prevent generation of display stains upon application of an external pressure.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls light transmittance of liquid crystal material using an electric field to display a picture. The liquid crystal displays are largely classified into a vertical electric field type and a horizontal electric field type depending on a direction that the electric field drives the liquid crystal material.
The vertical electric field applying type LCD drives a liquid crystal material in a twisted nematic (TN) mode where a vertical electric field is formed between a pixel electrode and a common electrode arranged opposite to each other on the upper and lower substrates. The vertical electric field applying type LCD has an advantage of a large aperture ratio while having a drawback of a narrow viewing angle of about 90°. The horizontal electric field applying type LCD drives a liquid crystal material in an in-plane switch (IPS) mode where a horizontal electric field is formed between the pixel electrode and the common electrode arranged in parallel to each other on the lower substrate. The horizontal electric field applying type LCD has an advantage of a wide viewing angle of about 160° while having a drawback of low aperture ratio and transmittance.
Recently, in order to overcome the disadvantage of the horizontal electric field applying type LCD, there has been suggested a liquid crystal display of a fringe field switching (hereinafter, referred to as “FFS”) type operated by a fringe field.
FIG. 1 shows a pixel area P in a FFS type liquid crystal display. Referring to FIG. 1, the FFS type liquid crystal display includes a data line DL and a gate line GL formed to cross each other, a thin film transistor (hereinafter, referred to as “TFTs) connected to a crossing of the data and gate lines DL and GL, a pixel electrode 16 provided at a pixel area P defined by the crossing structure of the data and gate lines DL and GL and connected to the TFT, a common electrode 2 provided at the pixel area P to form a fringe field together with the pixel electrode 16, and a common line 6 connected to the common electrode 2. The pixel electrode 16 includes a plurality of finger portions 16b and connecting portions 16a for commonly connecting the finger portions 16b at opposite ends of the finger portions 16b. The common electrode 2 is formed in a plate shape under the pixel electrode 16, with a gate insulating film and a protective film interposed therebetween, to form a fringe filed together with the finger portions 16b of the pixel electrode 16.
Such a FFS type liquid crystal display divides the pixel area P for displaying images into two domains D1 and D2 in order to prevent image distortion caused by viewing angle dependency. The pixel electrode is bent at a domain borderline B that separates the first and second domains D1 and D2 from each other. An electrode angle θ formed between the pixel electrode and a normal line (N) perpendicular to the domain borderline B is equal at each of the domains D1 and D2. This electrode angle θ is determined by initial design values. A driving voltage for driving liquid crystal can be reduced because the smaller the electrode angle θ, the larger the movement angle of the liquid crystal with respect to an applied voltage. And, the average director of the liquid crystal at low-transmittance areas of the electrode and the electrode gap center is close to 45°, and hence the brightness can be increased. Accordingly, there is a recent trend towards reducing the electrode angle θ to less than 15° in order to decrease driving voltage and increase brightness.
However, if the electrode angle θ is reduced, the liquid crystal collapses because it cannot maintain its original posture due to an external pressure, thus producing display stains as shown in FIG. 2. The electrode angle θ is deeply related to a torque of the liquid crystal as shown in the following Table 1. The smaller the electrode angle, the smaller the torque of the liquid crystal. Thus, it is difficult for liquid crystals to maintain orientation relative to their posture against an external force.
TABLE 1AngleTorque (a.u.)10 degrees0.3415 degrees0.5020 degrees0.6425 degrees0.7730 degrees0.87
Accordingly, as shown in FIG. 3, when an external pressure is applied to a display panel in a white gray scale, if a torque (To) caused by the external force is larger than an intrinsic torque (Tlc) of the liquid crystal, the orientation of the liquid crystal at the portion where the external pressure is applied cannot restore the original state but is kept in the state in which the external pressure is applied. This brings about domain differences with a portion where no external pressure is applied (a portion where an external pressure is applied is denoted by domain 1, and a portion where no external pressure is applied is denoted by domain 2), thereby causing a difference in color range depending on a viewing angle. FIG. 4 shows an example in which, if an external pressure is applied to the display panel in a white gray scale, luminance is reduced due the collapse of the border area between the domains in the external-pressure-applied area and thus display stains appear.